1. Field of the Invention
The present invention relates to a liquid crystal display, and more specifically to an alignment layer for initial alignment of a liquid crystal in a liquid crystal display.
2. Discussion of the Related Art
Ultra-thin flat panel displays include a display screen with a thickness of not more than a few centimeters. Of these, liquid crystal displays are currently used in a wide range of applications, such as notebook computers, monitors, spacecrafts and aircraft, in terms of advantages of low power consumption due to low operating voltage and ease of portability.
A general liquid crystal display includes a color filter substrate having a color filter layer formed thereon, a thin film transistor substrate arranged opposite to the color filter substrate and having thin film transistors formed thereon, and a liquid crystal layer interposed between the two substrates.
The alignment direction of the liquid crystal layer in the liquid crystal display is varied depending on an applied voltage and thus the light transmittance is controlled, thereby displaying images on a screen. To apply a voltage, electrodes are formed on the thin film transistor substrate and the color filter substrate. Specifically, pixel electrodes are arranged on the thin film transistor substrate, and a common electrode is arranged on the color filter substrate. A vertical electric field may be applied between the two substrates (in the case of a twisted nematic (TN) mode), or a horizontal electric field may be applied after the pixel electrodes and the common electrode are arranged parallel to the thin film transistor substrate (in the case of an in-plane switching (IPS) mode).
FIG. 1 is an exploded perspective view of a general TN mode liquid crystal display.
Referring to FIG. 1, gate lines 12 and data lines 14 crossing each other are formed on a thin film transistor substrate 10. Thin film transistors (T) are formed at crossings of the gate and data lines and pixel electrodes 16 are connected to the thin film transistors. In addition, a light-blocking layer 22 is formed on a color filter substrate 20 to prevent light from leaking, an RGB color filter layer 24 is formed on the light-blocking layer 22, and a common electrode 25 is formed thereon. In the case of an IPS mode, the common electrodes and the pixel electrodes are formed on the same substrate.
When a vertical electric field is generated between the pixel electrodes 16 formed on the thin film transistor substrate 10 and the common electrode 25 formed on the color filter substrate 20, the alignment direction of a liquid crystal is controlled.
Thereafter, the substrates 10 and 20 are attached to each other to form one liquid crystal panel in which a liquid crystal layer is formed between the substrates 10 and 20.
If the liquid crystal layer is randomly arranged between the substrates 10 and 20, liquid crystal molecules included in the liquid crystal layer are not arranged in a fixed direction. Although not shown in the figure, an alignment layer for initial alignment of the liquid crystal is formed between the thin film transistor substrate 10 and the color filter substrate 20.
Formation of such an alignment layer for initial alignment of a liquid crystal has predominantly been achieved by rubbing alignment.
This rubbing alignment includes the steps of applying an organic polymer, such as polyimide, to a substrate to form a thin film, curing the thin film, and rolling a rubbing roll covered with a rubbing cloth to rub the thin film organic polymer and to arrange side chains of the organic polymer in a particular direction.
A liquid crystal is aligned in the direction that the side chains of the organic polymer are arranged by the rubbing alignment. That is, the movement direction of the rubbing roll is the same as the alignment direction of the liquid crystal.
However, rubbing alignment has the following disadvantages.
Firstly, when the arrangement of a rubbing cloth is non-uniform, light leakage may occur. FIG. 2 is a perspective view schematically illustrating a state wherein the arrangement of a rubbing cloth is non-uniform.
As described above, elements such as thin film transistors, color filter layers and electrode layers are formed on a substrate. As illustrated in FIG. 2, when a rubbing roll 30 is rolled on the structures formed on a substrate 20 or 30, parts 32a of a rubbing cloth 32 covering the rubbing roll 30 may be non-uniformly arranged. This non-uniform arrangement of the rubbing cloth 32 causes a non-uniform array of side chains of an organic polymer in regions of the substrate rubbed by the parts 32a of the rubbing cloth. As a result, the alignment of the liquid crystal is not uniform, thus causing light leakage.
Secondly, when a rubbing cloth does not come into contact with a substrate, light leakage may occur. FIG. 3 is a perspective view schematically illustrating liquid crystal arrangement where a rubbing cloth does not come into contact with a substrate.
As explained earlier, electrode layers, such as pixel electrodes and a common electrode, are formed on a substrate. Due to a step height in electrode layers formed on a substrate 10, as illustrated in FIG. 3, a region (region “A”) is formed where a rubbing cloth 32 does not come into contact with the substrate 10. In this case, the alignment of a liquid crystal is not uniform in the region (“A”), resulting in light leakage.
In conclusion, according to a related art rubbing alignment method, when the arrangement of a rubbing cloth is non-uniform or a rubbing cloth does not come into contact with a substrate, rubbing cannot be performed well, causing the problem of light leakage. Thus, there is a need for a novel liquid crystal alignment method to solve the problems of the related art rubbing alignment method.
The above-mentioned problems of the related art rubbing alignment method are attributed to physical contact between a rubbing roll and a substrate.